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Effect of Substituents on Ultraviolet Absorption Spectra of Derivatives of O-Hydroxybenzophenone

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Effect of Substituents on Ultraviolet Absorption Spectra of Derivatives of O-Hydroxybenzophenone 508 CHEMICKÉ ZVESTI 22, 508—513 (19C8) Effect of Substituents on Ultraviolet Absorption Spectra of Derivatives of o-Hydroxybenzophenone P. HRDLOVIC, D. BELLUŠ Polymer Institute, Slovak Academy of Sciences, Bratislava UV absorption spectra of 14 derivatives of o-hydroxybenzophenone in ionized and unionized form in aqueous ethanol and in unionized form in hexane have been measured. Influence of substituents and solvents on the absorption band with the longest wavelength was discussed. The shift of this band under the influence of substituents in the unionized form was corre­ lated with Hammett's a constants. Derivatives of o-hydroxybenzophenone are known as efficient photostabilizers. High absorption efficiency in the near ultraviolet region is a fundamental prerequisite for this application. Absorption spectra of derivatives of o-hydroxybenzophenone were measured by several authors [1—3]. Recently W. L. Dilling [4] investigated the influence of solvents on the shift of absorption bands in benzophenone and its hydroxy derivatives. The purpose of this work is to follow the influence of substituents at positions 4 and 5 in o-hydroxybenzophenone and solvents at the absorption band with the longest wavelength. Experimental Ultraviolet absorption spectra were measured on a VSU-1 manual spectrophotometer of Carl Zeiss, Jena. The accuracy of reading of the absorption maxima was + 2 mm. The equipment was calibrated on the lines of a mercury high-pressure arc HQE 40. Derivatives of o-hydroxybenzophenone used for measurement were prepared accord­ ing to [5]. Ethanol and hexane used as solvents were spectrally pure. Hydrochloric acid and sodium hydroxide were analytically pure. Results and discussion Determination of the nature of the respective transitions of derivatives of 2-hydro- xybenzophenone is empirically possible on the basis of benzophenone and its hydroxy derivatives. Benzophenone displays in the ultraviolet region two absorption bands. The ab­ sorption band with Яшах = 254 nm in ethanol is attributed to the TI^TI* transition and the second with Amax = 334 nm in ethanol is attributed to the n -> тс* transition this being indicated by its low intensity, hypsochronic shift with growing solvent polarity and by its fine structure in non-polar solvents. In ^-hydroxybenzophenone the band Amax = 246 nm in ethanol belongs to the п^~тс* transition and the second absorption band Amax = 293 nm according to G. Porter [6—8] was related to an intramolecular charge transfer and was marked #-CT. In o-hydroxybenzophenone the band Amax = 260 nm in ethanol is again TI^TZ* and the second Amax = 337 nm in ethanol was marked by W. L. Dilling [4] o-CT. This designation of different Derivatives of o-Hydroxybenzophenone 509 absorption bands of derivatives of o-rrydroxybenzophenone was used in Table 1, quoting values of Amax of the respective derivatives in various solvents. Ultraviolet absorption spectra of derivatives of o-hydroxybenzophenone retain essentially the character of the spectrum of o-hydroxybenzophenone. The absorption band about 260 nm is moderately influenced by substituents of derivatives of o-hydroxybenzophenone. More markedly affected by substituents is the absorption band with the longest wavelength, which is shifted in the region 310—380 nm. Table 1 Ultraviolet spectra of derivatives of o-hydroxybenzophenone 0.1 M-HC1 O.lM-NaOH 50 vol. % of 50 vol. % of Hexane ethanol ethanol R я -» я* o-CT я -> я* я -> я* O-CT nm nm nm nm nm nm 1. H 203 337 242 382 258 338 256 2. 5-N02 311 245 401 252 315 267 287 3. 5-COCH3 259 320* 246 326 244 335 258 4. o-COC6H5 258 325 248 350 252 337 280a 5. 5-C1 260 345 245 395 258 349 6. 5-CH3 263 349 243 398 255 352 7. 5-OCH3 255 370 245 410 251 372 8. 5-OH 256 380 d d 9. 4-N0 271 348 241 430 271 359 2 259 10. 4-Cl 271 329 244 380 271 331 276 338 242 382 253 338 11. 4-CH3 270 242 325 242 372 241 326 12. 4-OCH3 291Ď 275rt 283Ď 245 320 345 d d 13. 4-OH 290Ď 269 328 243 350 d d 14. 4-OCOCH3 a) Shoulder; b) Band belongs to p-CT; с) Compound oxidizes in alkaline solution by atmos­ pheric oxygene; d) Compound is insoluble in this solvent. In derivatives with a hydroxyl or alkoxyl group in position 4 the absorption band appears at about 290 nm, analogically as in^p-hydroxybenzophenone. In consequence of the molecule of o-hydroxybenzophenone not being plane [9—12] the different absorption bands can be attributed to the respective parts of the molecule as follows: The absorption band with Лщах around 260 nm is caused by the phenylcarbonyl group. The absorption band with the longest wavelength is produced by interaction 510 P. Hrdloviö, D. Belluá of the electron-donor hydroxyl group in ortho position with the electron acceptor carbonyl group in excited state according to equation: * ° *°\\ °~ The absorption band with Amax around 290 nm is caused by analogical interaction of the electron-donor group in £>ara-position. Table 2 Influence of substituents on the absorption band with the longest wavelength in derivatives of o-hydroxybenzophenone 3 1 — c R Гтах • Ю- [cm" ] OR (a) ^OH ^co ( ) 1. H 29.67 0 0 2. 5-N02 32.15 0.64 0.56 3. 5-СОСНз 31.25 0.60 0.21 4. 5-Cl 28.99 —0.24 —0.14 5. 5-CH3 28.63 —0.13 —0.08 6. 5-OCH3 27.03 —0.50 (b) —0.25 { 7. 5-OH 26.31 —0.61 (6) —0.36 8. 4-N02 28.74 0.08 —0.09 9. 4-C1 30.40 —0.10 0.14 10. 4-CH3 29.59 —0.02 0.09 11. 4-OH 31.25 —0.25 0.36 12. 4-OCH3 30.77 —0.11 0.39 a) Ref. [15]; b) Ref. [16]; с) Ref. [5]. The effect of substituents on physical chemical magnitudes in series of structurally related compounds are quantitatively characterized by Hammett's a constants. Correlations of shifts of absorption bands in electron spectra with a constant have limited validity only, even though there were established in certain cases satisfactory linear correlations. For derivatives of o-hydroxybenzophenone an equation put forward by С N. R. Rao [13, 14] was used: -1 where imax is the position of the maximum of the absorption band in cm , <*R is the resonance part of Hammett's constant defined on the basis of phenol dissocia­ tion constants. If we choose for derivatives substituted at position 5, o*Ř for the ^am-position and for derivatives substituted at position 4, o*Ř for the wiefo-position we obtain satisfactory correlations (Fig. 1 and 2). It is a shortcoming of these corre­ lations that the gradient has a different sign for derivatives substituted at position 5 (o = 4320) and at position 4 (0 = —7760). Derivatives of o-Hydroxybenzophenone 511 -025 OL Ol" 025 Fig. 1. Dependence of the jWx on a^ for Fig. 2. Dependence of the £max on o^ for derivatives of o-hydroxybenzophenone derivatives of o-hydroxybenzophenone substituted at position 5. substituted in position 4. Correlation constants calculated by the Correlation constants: о = —7760, v0 = least squares method are slope Q = 4320 = 29 550, r = 0.98, s = 280. (Numbers intercept, vo = 29 290, correlation coeffi­ correspond to the sequence in Table 2.) cient r = 0.99, standard deviation s = = 240. (Numbers correspond to the sequence in Table 2.) - aco Fig. 3. Dependence of the řmax on Оон: for derivatives of o-hydroxybenzophenone. Correlation constants: Q = 4840, v0 = = 29 420, s = 740, r = 0.85. (Numbers correspond to the sequence in Table 2.) A more general correlation is obtained in case of use of the difference of the given substituent with regard to the hydroxyl o"OH and the carbonyl group aco in the form í'max = V0 + (oröH — ffco)- This dependence is more general because it is valid for derivatives substituted in position 4 and 5 (Fig. 3). Both relations indicate that regarding the influence of substituents on the shift of absorption bands above all their resonance effect becomes apparent. 512 P. Hrdlovič, D. Belluš The absorption band with the longest wavelength is shifted bathochromically if there are in para-position against one another on the trisubstituted benzene nucleus of derivatives of o-hydroxybenzophenone substituents of the same type. For instance in derivates substituted at position 4 it can be observed the bathochromic shift if in ^ara-position opposite to the electron-acceptor carbonyl group there is an electron- acceptor group. In derivatives substituted at position 5, at the para-position opposite to the electron-donor hydroxyl group there should be an electron-donor group. A similar effect was observed in derivatives of o-nitrophenol substituted at posi­ tion 4 and 5 for the absorption band with the longest wavelength [17]. Solvents with different polarity as f. i. ethanol and hexane have a relatively small influence on the shift of the absorption band with the longest wavelength, only, this being in accord with, measurements of W. L. Dilling [4]. In the ionized form of derivatives of o-hydroxybenzophenone two bands were observed in the absorption spectra. The absorption band w'th Amax of about 245 nm is nearly constant for all derivatives. The absorption band with the longest wave­ length is bathochromically shifted against the non dissociated form. It belongs to the group 0" and is influenced by substituents. However, we were not succesful in qualitative and quantitative characterization of the influence of substituents on its shifts. A small bathochromic shift was observed in derivatives with the substituents —COCH3 and — COC6H5 in the dissociated form in comparison with the non disso­ ciated one, but the extinction coefficient has changed hyperchromically.
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